FIG. 1 shows an example of the construction of a conventional head-mounted display (HMD), which is one type of virtual reality device. In order for an eye 3 of an observer to view a virtual image, an actual image is displayed on a liquid crystal panel 1, which is illuminated by a backlight 4. In FIG. 1 the virtual image is viewed at infinity by the observer's eye 3 because the liquid crystal panel 1 is disposed at the focal point position of the lens 2. In general, head-mounted devices are constructed so as to form an image at a distance in the range of 25 cm to infinity.
It has been proposed to allow a changing of the positions of the liquid crystal panel 1 and the lens 2 so as to adjust the position of the image to match the diopter of the observer 3 (first conventional example). Japanese Laid-Open Patent Application No. 7-30928 discloses an arrangement wherein the diopter is changed by moving the focal point in conjunction with the perspective of an image so as to controllably move the image display device (i.e., liquid crystal panel 1) forwarde and backwarde in accordance with the parallax of the displayed image (second conventional example).
However, the first conventional example normally provides that an observer continues to view the image at a fixed position insofar as the observer cannot himself optionally change the position of the image. Particularly when this viewing state continues over a long period of time, the observer's eyes become fatigued so as to cause a concern as to possible vision impairment. The second conventional example of a display described above is capable of reducing eye fatigue to some degree, but requires a complex construction which is necessary to control the position of the image display device in accordance with the displayed image. Furthermore, when the image parallax does not change, the image continues to be viewed at the same image position. In this situation, the resulting eye fatigue is similar to that of the first conventional example.